U.S. patent number 7,211,352 [Application Number 10/750,152] was granted by the patent office on 2007-05-01 for single ion conductor-containing composite polymer electrolyte for lithium secondary battery and method of manufacturing the same.
This patent grant is currently assigned to Electronics and Telecommunications Research Institute. Invention is credited to Soon Ho Chang, Young Gi Lee, Kwang Sun Ryu.
United States Patent |
7,211,352 |
Lee , et al. |
May 1, 2007 |
Single ion conductor-containing composite polymer electrolyte for
lithium secondary battery and method of manufacturing the same
Abstract
Provided are a composite polymer electrolyte for a lithium
secondary battery that includes a composite polymer matrix
structure having a single ion conductor-containing polymer matrix
to enhance ionic conductivity and a method of manufacturing the
same. The composite polymer electrolyte includes a first polymer
matrix made of a first porous polymer with a first pore size; a
second polymer matrix made of a single ion conductor, an inorganic
material, and a second porous polymer with a second pore size
smaller than the first pore size. The second polymer matrix is
coated on a surface of the first polymer matrix. The composite
polymer matrix structure can increase mechanical properties. The
single ion conductor-containing porous polymer matrix of a
submicro-scale can enhance ionic conductivity and the
charge/discharge cycle stability.
Inventors: |
Lee; Young Gi (Daejeon,
KR), Ryu; Kwang Sun (Daejeon, KR), Chang;
Soon Ho (Daejeon, KR) |
Assignee: |
Electronics and Telecommunications
Research Institute (KR)
|
Family
ID: |
33297360 |
Appl.
No.: |
10/750,152 |
Filed: |
December 30, 2003 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20040214089 A1 |
Oct 28, 2004 |
|
Foreign Application Priority Data
|
|
|
|
|
Apr 25, 2003 [KR] |
|
|
10-2003-0026420 |
|
Current U.S.
Class: |
429/309; 429/314;
429/317; 429/320; 429/322; 429/323; 429/321; 429/319; 429/316;
429/310; 429/306 |
Current CPC
Class: |
H01M
10/0565 (20130101); Y02E 60/10 (20130101); H01M
2300/0094 (20130101); H01M 10/052 (20130101); H01M
2300/0082 (20130101) |
Current International
Class: |
H01M
10/40 (20060101); H01M 4/66 (20060101); H01M
6/18 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
www.smallparts.com/products/descriptions (porous pvdf and porous
polyethylene). cited by examiner .
www.osmolabstore.com (OEM GE PVDF (polyvinylidene fluoride)
Transfer Membranes) (porous pvdf). cited by examiner .
Performance of Bellcore's plastic rechargeable Li-ion batteries,
J.M. Tarascon, A.S. Gozdz, C. Schmutz, F. Shokoohi, P.C. Warren,
Solid State Ionics 86-88 (1996), pp. 49-54. cited by other .
J. Electrochem. Soc., vol. 142, No. 6, Jun. 1995, pp. 1789-1798.
cited by other.
|
Primary Examiner: Ryan; Patrick J.
Assistant Examiner: Martin; Angela J.
Attorney, Agent or Firm: Blakely Sokoloff Taylor &
Zafman
Claims
What is claimed is:
1. A composite polymer electrolyte for a lithium secondary battery,
which comprises: a first polymer matrix made of a first porous
polymer with a first pore size, wherein the first porous polymer is
polyethylene, polypropylene, polyimide, polysulfone, polyurethane,
polyvinyichioride, cellulose, nylon, polyacrylonitrile,
polyvinylidene fluoride, polytetrafluoroethylene, a copolymer or
blend thereof, and wherein the first polymer matrix does not
comprise a polymer type single ion conductor; a second polymer
matrix coated on the first polymer matrix and made of a single ion
conductor consisting essentially of polymer, an inorganic material,
and a second porous polymer with a second pore size smaller than
the first pore size, wherein the second porous polymer is a
vinylidene fluoride based polymer, an acrylate based polymer, a
copolymer or a blend thereof, and wherein the second polymer matrix
has an ionic conductivity equal to or higher than the ionic
conductivity of the first polymer matrix; and an electrolyte
solution impregnated into the first polymer matrix and the second
polymer matrix.
2. The composite polymer electrolyte of claim 1, wherein the single
ion conductor is perfluorinated ionomer,
methylmethacrylate/alkaline metal methacrylate copolymer ionomer,
methylmethacrylate/alkaline itaconate copolymer ionomer,
methylmethacrylate/alkaline maleate copolymer ionomer, polystyrene
ionomer, or a blend thereof.
3. The composite polymer electrolyte of claim 1, wherein the second
porous polymer is a copolymer of vinylidene fluoride and
hexafluoropropylene, a copolymer of vinylidene fluoride and
trifluoroethylene, a copolymer of vinylidene fluoride and
tetrafluoroethylene, polymethylacrylate, polyethylacrylate,
polymethylmethacrylate, polyethylmethacrylate, polybutylacrylate,
polybutylmethacrylate, polyvinylacetate, polyethylene oxide,
polypropylene oxide, a copolymer or blend thereof.
4. The composite polymer electrolyte of claim 1, wherein the
inorganic material is selected from the group consisting of silica,
talc, alumina (Al.sub.2O.sub.3), .gamma.LiAlO.sub.2, TiO.sub.2,
zeolite, molybdenum phosphate hydrate, and tungsten phosphate
hydrate.
5. The composite polymer electrolyte of claim 1, wherein the
inorganic material is added in an amount of 1 to 100% by weight,
based on the total weight of the polymer of the second porous
matrix.
6. The composite polymer electrolyte of claim 1, wherein the first
polymer matrix has a thickness of 10 to 25 .mu.m and the second
polymer matrix has a thickness of 0.5 to 10 .mu.m.
7. The composite polymer electrolyte of claim 1, wherein the
electrolyte solution is made of ethylene carbonate, propylene
carbonate, dimethyl carbonate, diethyl carbonate, methylethyl
carbonate, tetrahydrofuran, 2-methyltetrahydrofuran,
dimethoxyethane, methyl formate, ethyl formate,
gamma-butyrolactone, or a mixture thereof.
8. The composite polymer electrolyte of claim 1, wherein the
electrolyte solution is impregnated into the first polymer matrix
and the second polymer matrix in an amount of 1 to 1,000% by
weight, based on the total weight of the polymer of the first
polymer matrix and the second polymer matrix.
9. The composite polymer electrolyte of claim 1, wherein the
electrolyte solution comprises at least one lithium salt selected
from the group consisting of lithium perchlorate (LiClO.sub.4),
lithium triflate (LiCF.sub.3SO.sub.3), lithium hexafluorophosphate
(LiPF.sub.6), lithium tetrafluoroborate (LiBF.sub.4), and lithium
trifluoromethanesulfonylimide (LiN(CF.sub.3SO.sub.2).sub.2).
10. The composite polymer electrolyte of claim 9, wherein the
lithium salt is dissolved in the electrolyte solution in an amount
of 1 to 200% by weight, based on the total weight of the polymer of
the first polymer matrix and the second polymer matrix.
11. A method of manufacturing a composite polymer electrolyte for a
lithium secondary battery, the method comprising: preparing a first
polymer matrix made of a first porous polymer with a first pore
size, wherein the first porous polymer is polyethylene,
polypropylene, polyimide, polysulfone, polyurethane,
polyvinylchloride, cellulose, nylon, polyacrylonitrile,
polyvinylidene fluoride, polytetrafluoroethylene, a copolymer, or a
blend thereof, and wherein the first polymer matrix does not
comprise a polymer single ion conductor; uniformly dissolving a
single ion conductor consisting essentially of polymer, an
inorganic material, and a second porous polymer with a second pore
size smaller than the first pore size in a co-solvent in a
predetermined ratio to produce a solution, wherein the second
porous polymer is a vinylidene fluoride based polymer, an acrylate
based polymer, a copolymer, or a blend thereof; coating the first
polymer matrix with the solution to form a second polymer matrix on
the first polymer matrix, wherein the second polymer matrix has an
ionic conductivity equal to or higher than the ionic conductivity
of the first polymer matrix; and impregnating the first polymer
matrix and the second polymer matrix with an electrolyte
solution.
12. The method of claim 11, wherein the co-solvent is selected from
the group consisting of ethanol, methanol, isopropyl alcohol,
acetone, dimethylformamide, dimethylsulfoxide, N-methylpyrrolidone,
and a mixture thereof.
Description
BACKGROUND OF THE INVENTION
This application claims the priority of Korean Patent Application
No. 2003-26420, filed on Apr. 25, 2003, in the Korean Intellectual
Property Office, the disclosure of which is incorporated herein in
its entirety by reference.
1. Field of the Invention
The present invention relates to a polymer electrolyte for a
lithium secondary battery and a method of manufacturing the same.
More particularly, the present invention relates to a composite
polymer electrolyte for a lithium secondary battery, which includes
a composite polymer matrix structure comprised of two porous
polymer matrices of different pore sizes impregnated with an
electrolyte solution, and a method of manufacturing the same.
2. Description of the Related Art
Recently, as electric, electronic, communication, and computer
industries are rapidly expanding, demands for secondary batteries
with high performance and high stability have increased. In
particular, as electronic devices progressively become small, thin,
and lightweight, in the office automation field, desktop computers
are being replaced with laptop or notebook computers that are
smaller and lighter than the desktop computers. Also, the use of
portable electronic devices such as camcorders and cellular phones
has rapidly grown.
As electronic devices become small, thin, and lightweight,
secondary batteries that are used as power supply sources for the
electronic devices are also required to have higher performance.
For this, lithium secondary batteries to replace conventional lead
storage batteries or lithium-cadmium batteries have been actively
researched and developed to satisfy the requirements of small-size,
lightness, high energy density, and frequent charging
operations.
The lithium secondary batteries include a cathode and an anode made
of an active material that can induce intercalation and
de-intercalation of lithium ions. An organic electrolyte or a
polymer electrolyte that allows for the movement of the lithium
ions is interposed between the cathode and the anode. The lithium
secondary batteries generate electric energy by oxidation/reduction
due to intercalation/de-intercalation of the lithium ions in the
cathode and the anode.
The cathode of the lithium secondary batteries has a potential
higher than the electrode potential of lithium, by as much as about
3 to 4.5 V, and is mainly made of complex oxide of lithium with
transition metal for intercalation/de-intercalation of the lithium
ions. For example, lithium cobalt oxide (LiCoO.sub.2), lithium
nickel oxide (LiNiO.sub.2), and lithium manganese oxide
(LiMnO.sub.2) are mainly used as a cathode material. On the other
hand, the anode is mainly made of a lithium metal, a lithium alloy,
or a carbonaceous material that exhibits a chemical potential
similar to the lithium metal upon the
intercalation/de-intercalation of the lithium ions, so as to
reversibly receive or emit the lithium ions while maintaining
structural and electrical properties.
The lithium secondary batteries are classified into lithium ion
batteries (LIBs) and lithium polymer batteries (LPBs) according to
the types of electrolytes. While the lithium ion batteries use a
liquid electrolyte/separation film system, the lithium polymer
batteries use a polymer electrolyte. In particular, the lithium
polymer batteries can be sub-classified into lithium metal polymer
batteries (LMPBs) using a lithium metal as an anode and lithium ion
polymer batteries (LIPBs) using carbon as the anode. In the lithium
ion batteries using a liquid electrolyte, there arise problems due
to instability of the liquid electrolyte. Although alternatives
such as use of an electrode material capable of compensating for
the instability of the liquid electrolyte or installation of a
safety apparatus can be considered, a manufacture cost increases
and it is difficult to increase the capacity of the batteries. On
the contrary, the lithium polymer batteries have many advantages
such as low manufacture cost, diversity of size and shape, and high
voltage and large capacity by lamination. Therefore, attention has
been paid to the lithium polymer batteries as next generation
batteries.
In order for the lithium polymer batteries to be commercially
available, the polymer electrolyte must satisfy the requirements
such as excellent ionic conductivity, mechanical properties, and
interfacial stability between it and electrodes. In particular, in
the lithium metal polymer batteries, dendritic growth of lithium on
a lithium anode, formation of dead lithium, or interfacial
phenomenon between the lithium anode and the polymer electrolyte
adversely affects the stability and cycle characteristics of the
batteries. In view of these problems, various polymer electrolytes
have been developed.
At an initial stage for developments of polymer electrolytes,
solventless polymer electrolytes had been mainly studied. The
solventless polymer electrolytes are manufactured by dissolving a
mixture of a salt with polyethylene oxide or polypropylene oxide in
a co-solvent, followed by casting (see EP78505 and U.S. Pat. No.
5,102,752). However, the solventless polymer electrolytes have very
low ionic conductivity at room temperature.
Gel polymer electrolytes are another example of the polymer
electrolytes. The gel polymer electrolytes have high ionic
conductivity of more than 10.sup.-3 S/cm, and are manufactured in
the form of a film after dissolving a salt and a common polymer
such as polyacrylonitrile, polymethylmethacrylate,
polyvinylchloride, and polyvinylidene fluoride in an organic
solvent such as ethylene carbonate and propylene carbonate and a
co-solvent [K. M. Abraham et al., J. Electrochem. Soc., 142, 1789,
1995]. However, these gel polymer electrolytes have automation
process-related problems such as deterioration of mechanical
properties due to the used organic solvent, a need of a specific
process condition when actually used for the lithium polymer
batteries, and removal of the co-solvent.
Recently, there is disclosed a method of manufacturing lithium
secondary batteries, which includes: preparing a porous polymer
matrix, laminating a cathode, the porous polymer matrix, and an
anode to produce a laminate, and impregnating the laminate with an
electrolyte solution [J. M. Tarascon et al., Solid State Ionics, 86
88, 49, 1996; and U.S. Pat. No. 5,456,000]. In this case, although
ionic conductivity is slightly enhanced, mechanical properties are
little enhanced.
In spite of numerous attempts to improve the physicochemical
properties of polymer electrolytes as described above, current
polymer electrolytes still exhibit low ionic conductivity and
insufficient mechanical properties, as compared to the electrolyte
solution/separation film system of the lithium ion batteries. This
is because due to compatibility between a polymer matrix and an
electrolyte solution, an electrolyte film becomes flexible as
impregnation of the polymer matrix with the electrolyte solution
increases. Also, since the electrolyte film has more compact
microporous morphology relative to the separation film, an ion
transfer path is curved, and thus, an ion transfer distance becomes
long. For this reason, the lithium metal polymer batteries exhibit
drastically low ionic conductivity, relative to the lithium ion
batteries, even though dendritic growth of lithium at a surface of
a lithium anode is slightly prevented. Therefore, thin film
formation for the polymer electrolyte is difficult and the total
resistance of batteries is increased, thereby deteriorating
charge/discharge cycle performance.
SUMMARY OF THE INVENTION
The present invention provides a thin film, composite polymer
electrolyte for a lithium secondary battery, which has improved
ionic conductivity and mechanical properties.
The present invention also provides a simplified method of
manufacturing the thin film, composite polymer electrolyte for a
lithium secondary battery.
According to an aspect of the present invention, there is provided
a composite polymer electrolyte for a lithium secondary battery,
which comprises: a first polymer matrix made of a first porous
polymer with a first pore size; a second polymer matrix coated on
the first polymer matrix and made of a single ion conductor, an
inorganic material, and a second porous polymer with a second pore
size smaller than the first pore size; and an electrolyte solution
impregnated into the first polymer matrix and the second polymer
matrix. The first polymer matrix may have a thickness of 10 to 25
.mu.m and the second polymer matrix may have a thickness of 0.5 to
10 .mu.m.
The single ion conductor may be perfluorinated ionomer,
methylmethacrylate/alkaline metal methacrylate copolymer ionomer,
methylmethacrylate/alkaline itaconate copolymer ionomer,
methylmethacrylate/alkaline maleate copolymer ionomer, polystyrene
ionomer, or a blend thereof.
According to another aspect of the present invention, there is
provided a method of manufacturing a composite polymer electrolyte
for a lithium secondary battery. A first polymer matrix made of a
first porous polymer with a first pore size is prepared. A single
ion conductor, an inorganic material, and a second porous polymer
with a second pore size smaller than the first pore size are
dissolved in a co-solvent in a predetermined ratio to produce a
solution. The first polymer matrix is coated with the solution to
form a second polymer matrix on the first polymer matrix. The first
polymer matrix and the second polymer matrix are impregnated with
an electrolyte solution.
The polymer electrolyte for a lithium secondary battery according
to the present invention has excellent mechanical properties due to
a composite polymer matrix structure comprised of porous polymer
matrices of different pore sizes, and excellent ionic conductivity
due to a single ion conductor-containing porous polymer matrix of a
submicro-scale. Also, erosion of a lithium anode and dendritic
growth of lithium on a surface of the lithium anode are prevented,
thereby preventing a short-circuit of the battery. Furthermore, the
charge/discharge cycle performance and stability of a lithium metal
polymer secondary battery are remarkably enhanced. Still
furthermore, the polymer electrolyte of the present invention can
be manufactured in the form of an ultra-thin film and a manufacture
process is also simplified.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other features and advantages of the present
invention will become more apparent by describing in detail
exemplary embodiments thereof with reference to the attached
drawings in which:
FIG. 1 depicts a schematic structure of a composite polymer
electrolyte for a lithium secondary battery according to a
preferred embodiment of the present invention;
FIG. 2 is a flowchart that illustrates a process of manufacturing a
composite polymer electrolyte for a lithium secondary battery
according to a preferred embodiment of the present invention;
FIG. 3 is a graph showing ionic conductivities of composite polymer
electrolytes according to the present invention;
FIG. 4 is a graph showing charge/discharge characteristics of unit
batteries using composite polymer electrolytes according to the
present invention; and
FIG. 5 is a graph showing cycle performance of unit batteries using
composite polymer electrolytes according to the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a schematic sectional view that depicts a structure of a
composite polymer electrolyte for a lithium secondary battery
according to a preferred embodiment of the present invention.
Referring to FIG. 1, a composite polymer electrolyte 10 for a
lithium secondary battery according to the present invention
includes a first polymer matrix 12 made of a first porous polymer
with a first pore size and a second polymer matrix 14 coated on a
surface of the first polymer matrix 12. The second polymer matrix
14 is made of a single ion conductor, an inorganic material, and a
second porous polymer with a second pore size (submicro-scale)
smaller than the first pore size. Preferably, the first polymer
matrix 12 has a thickness of 10 to 25 .mu.m and the second polymer
matrix 14 has a thickness of 0.5 to 10 .mu.m.
The first polymer matrix 12 and the second polymer matrix 14 are
impregnated with an electrolyte solution 16.
The first porous polymer for the first polymer matrix 12 may be
polyethylene, polypropylene, polyimide, polysulfone, polyurethane,
polyvinylchloride, cellulose, nylon, polyacrylonitrile,
polyvinylidene fluoride, polytetrafluoroethylene, a copolymer or
blend thereof.
The single ion conductor for the second polymer matrix 14 may be
perfluorinated ionomer, methylmethacrylate/alkaline metal
methacrylate copolymer ionomer, methylmethacrylate/alkaline
itaconate copolymer ionomer, methylmethacrylate/alkaline maleate
copolymer ionomer, polystyrene ionomer, or a blend thereof.
The second porous polymer for the second polymer matrix 14 may be a
vinylidene fluoride based polymer, an acrylate based polymer, a
copolymer or blend thereof. Preferably, the second porous polymer
is a copolymer of vinylidene fluoride and hexafluoropropylene, a
copolymer of vinylidene fluoride and trifluoroethylene, a copolymer
of vinylidene fluoride and tetrafluoroethylene, polymethylacrylate,
polyethylacrylate, polymethylmethacrylate, polyethylmethacrylate,
polybutylacrylate, polybutylmethacrylate, polyvinylacetate,
polyethylene oxide, polypropylene oxide, a copolymer or blend
thereof.
The inorganic material for the second polymer matrix 14 may be
silica, talc, alumina (Al.sub.2O.sub.3), .gamma.-LiAlO.sub.2,
TiO.sub.2, zeolite, molybdenum phosphate hydrate, or tungsten
phosphate hydrate. The inorganic material may be added in an amount
of 1 to 100% by weight, preferably about 1 to 50% by weight, based
on the total weight of the polymer of the second polymer matrix
14.
The electrolyte solution 16 is impregnated into the first polymer
matrix 12 and the second polymer matrix 14 in an amount of 1 to
1,000% by weight, preferably about 1 to 500% by weight, based on
the total weight of the polymer of the first polymer matrix 12 and
the second polymer matrix 14.
The electrolyte solution 16 may be made of ethylene carbonate,
propylene carbonate, dimethyl carbonate, diethyl carbonate,
methylethyl carbonate, tetrahydrofuran, 2-methyltetrahydrofuran,
dimethoxyethane, methyl formate, ethyl formate,
gamma-butyrolactone, or a mixture thereof.
A lithium salt is dissolved in the electrolyte solution 16 in an
amount of about 1 to 200% by weight, preferably about 1 to 100% by
weight, based on the total weight of the polymer of the first
polymer matrix 12 and the second polymer matrix 14.
The lithium salt may be at least one selected from the group
consisting of lithium perchlorate (LiClO.sub.4), lithium triflate
(LiCF.sub.3SO.sub.3), lithium hexafluorophosphate (LiPF.sub.6),
lithium tetrafluoroborate (LiBF.sub.4), and lithium
trifluoromethanesulfonylimide (LiN(CF.sub.3SO.sub.2).sub.2).
FIG. 2 is a flowchart that illustrates a process of manufacturing a
composite polymer electrolyte for a lithium secondary battery
according to a preferred embodiment of the present invention.
Referring to FIGS. 1 and 2, first, the first polymer matrix 12 made
of the first porous polymer with micro-scale morphology is formed
to a thickness of about 10 to 25 .mu.m (step 22).
Next, the single ion conductor, the inorganic material, and the
second porous polymer with submicro-scale morphology are uniformly
dissolved in a predetermined ratio in a co-solvent to produce a
solution (step 24). Here, the co-solvent may be selected from the
group consisting of ethanol, methanol, isopropyl alcohol, acetone,
dimethylformamide, dimethylsulfoxide, N-methylpyrrolidone, and a
mixture thereof.
The solution is coated on a surface of the first polymer matrix 12
to form the second polymer film 14 with a thickness of about 0.5 to
10 .mu.m (step 26). As a result, the composite polymer matrix
structure comprised of the first polymer matrix 12 and the second
polymer matrix 14 that are different in morphologies by different
pore sizes is produced.
Next, the first polymer matrix 12 and the second polymer matrix 14
are impregnated with the electrolyte solution 16 to complete the
composite polymer electrolyte 10 as shown in FIG. 1 (step 28).
Hereinafter, the method of manufacturing composite polymer
electrolytes for lithium secondary batteries according to the
present invention will be described more specifically by way of
Examples. It is, however, to be borne in mind that the following
Examples are provided only for illustrations and thus the present
invention is by no means limited to or by them.
EXAMPLE 1
In order to manufacture a composite polymer electrolyte for a
lithium secondary battery according to the process illustrated in
FIGS. 1 and 2, first, a copolymer of vinylidene fluoride and
hexafluoropropylene was dissolved with perfluorinated ionomer as a
single ion conductor in acetone/methanol used as a co-solvent to
obtain a solution containing 2% by weight of the copolymer. Then,
silica was added to the solution in an amount of 20% by weight,
based on the total weight of the copolymer. A dispersion thus
obtained was cast on a porous polyethylene film with a thickness of
25 .mu.m and the co-solvent was then evaporated. As a result, a
composite polymer matrix structure with different morphologies in
which a compact microporous polymer matrix was coated on a surface
of the porous polyethylene film was obtained. The obtained
composite polymer matrix structure was transferred into a glove box
of an argon atmosphere and then immersed in an electrolyte solution
in which 1M lithium hexafluorophosphate was contained in a mixture
solvent (1:1, molar ratio) of ethylene carbonate and dimethyl
carbonate to produce a polymer electrolyte.
EXAMPLE 2
A polymer electrolyte was manufactured in the same manner as in
Example 1 except that 5% by weight of a coating solution was
used.
EXAMPLE 3
A polymer electrolyte was manufactured in the same manner as in
Example 1 except that 10% by weight of a coating solution was
used.
EXAMPLE 4
A polymer electrolyte was manufactured in the same manner as in
Example 1 except that methylmethacrylate/alkaline metal
methacrylate copolymer ionomer was used instead of the
perfluorinated ionomer.
EXAMPLE 5
A polymer electrolyte was manufactured in the same manner as in
Example 1 except that 10% by weight of TiO.sub.2 was used instead
of the silica.
EXAMPLE 6
A polymer electrolyte was manufactured in the same manner as in
Example 1 except that a porous polytetrafluoroethylene film with a
thickness of 16 .mu.m was used instead of the porous polyethylene
film.
COMPARATIVE EXAMPLE
In order to perform characteristics comparison with the polymer
electrolytes obtained in Examples 1 through 6, a porous
polyethylene film with a thickness of 25 .mu.m was immersed in an
electrolyte solution in which 1M lithium hexafluorophosphate was
contained in a mixture solvent (1:1, molar ratio) of ethylene
carbonate and dimethyl carbonate, to produce a separation
film/liquid electrolyte system.
EXAMPLE 7
In order to measure a charge/discharge cycle, individual unit
batteries were manufactured using the composite polymer
electrolytes obtained in Examples 1, 2, and 3, and the separation
film/liquid electrolyte system obtained in Comparative Example.
There were used cathode plates made of a mixture of 80% by weight
of lithium-manganese-nickel oxide powders, 12% by weight of a
conductive agent, and 8% by weight of a binder. Lithium metal foils
were used as anode plates. Charge/discharge cycles were repeated in
such a way that a charge was carried out until 4.8 V and then a
discharge was carried out until 2.0 V, under a charge/discharge
current density of 1 mA (C/5 rate).
FIG. 3 is a comparative graph showing ionic conductivities of the
composite polymer electrolytes of the present invention and the
separation film/liquid electrolyte system of Comparative Example.
The composite polymer electrolytes of the present invention were
those obtained in Examples 1, 2, and 3.
As shown in FIG. 3, the individual polymer electrolytes obtained in
Examples 1, 2, and 3 exhibited similar or superior ionic
conductivities, as compared to Comparative Example.
FIG. 4 is a graph showing charge/discharge characteristics of unit
batteries using composite polymer electrolytes of the present
invention. In detail, FIG. 4 is a comparative graph showing initial
charge/discharge characteristics of unit batteries using the
polymer electrolytes obtained in Examples 1, 2, and 3, and the
separation film/liquid electrolyte system obtained in Comparative
Example.
As shown in FIG. 4, the unit batteries using the composite polymer
electrolytes of the present invention exhibited initial
charge/discharge characteristics similar to Comparative Example
commercially available. This result indicates that the initial
charge/discharge characteristics of the unit batteries using the
composite polymer electrolytes of the present invention are within
an acceptable range.
FIG. 5 is a graph showing cycle performance of unit batteries using
composite polymer electrolytes of the present invention. In detail,
FIG. 5 is a comparative graph showing the cycle performance of unit
batteries using the polymer electrolytes obtained in Examples 1, 2,
and 3 and the separation film/liquid electrolyte system obtained in
Comparative Example.
As shown in FIG. 5, the unit batteries using the composite polymer
electrolytes of the present invention exhibited the maintenance
ability of excellent discharge capacity, as compared to Comparative
Example.
As is apparent from the above description, the polymer electrolyte
for a lithium secondary battery according to the present invention
includes a composite polymer matrix structure. The composite
polymer matrix structure includes the first polymer matrix with
good mechanical properties, and the second polymer matrix with more
compact porous structure (submicro-scale) than the first polymer
matrix, coated on a surface of the first polymer matrix. The
composite polymer matrix structure has different morphologies by
different pore sizes, thereby providing enhanced mechanical
properties, as compared to a conventional gel polymer electrolyte.
The single ion conductor of the second polymer matrix with a
submicro-scale porous structure can remarkably enhance ionic
conductivity. Furthermore, erosion of a lithium anode and dendritic
growth of lithium on a surface of the lithium anode can be
prevented, thereby preventing a short-circuit of the battery. Still
furthermore, charge/discharge cycle performance and stability of a
lithium metal polymer secondary battery can be remarkably
enhanced.
In addition, the polymer electrolyte for a lithium battery of the
present invention can be manufactured in the form of an ultra-thin
film. Also, post-injection of the electrolyte solution can simplify
a manufacture process, thereby increasing a process yield.
While the present invention has been particularly shown and
described with reference to exemplary embodiments thereof, it will
be understood by those of ordinary skill in the art that various
changes in form and details may be made therein without departing
from the spirit and scope of the present invention as defined by
the following claims.
* * * * *
References